As known, most aircrafts, including civil transport airplanes, are provided with an autopilot enabling to check the trajectory and the speed according to instructions pre-established by the crew. Such an autopilot is a finite state automaton, each state corresponding to a particular piloting mode for the piloting control in the vertical plane and in the horizontal plane as well as for the speed holding.
It is further known that switching from one piloting mode to another one is generally achieved by a voluntary action of the crew on the control interfaces of the autopilot.
It is also known that, because of the existence of strong interactions between the different piloting modes, a simple action of the crew on the control interfaces of the autopilot could result in a cascade of mode transitions for the latter, such a cascade having consequences on the piloting control in the vertical plane and in the horizontal plane as well as on the speed holding of the aircraft.
Moreover, a voluntary action of the crew on other piloting controls (for instance the control lever of the airbrakes) than those associated with the autopilot, can also trigger mode transitions of the autopilot.
In some conditions, mode transitions can also occur automatically with no voluntary action of the crew, in order to protect the aircraft against flight field excursions.
Whatever the cause of the changes of piloting modes of the autopilot, the crew is notified about it by one or more visual alarms on the piloting interfaces, as well as by sound alarms.
Now, despite such visual and sound alarm systems, the crews may not become aware of at least one mode change of the autopilot of the aircraft (for instance because of a high or particularly stressful work load) and continue to act as if the autopilot was still in the desired piloting mode.
Moreover, even warned, the crews, being generally unaware of the triggering conditions for the mode transitions, find it hard to react to such mode changes, which may lead to critical situations where the crews become, for instance, aware that the autopilot carries out unwanted actions; however, such crews do not generally know the procedure to be applied in order to switch back to the desired piloting mode.
Furthermore, an undetected and un-notified defect of the autopilot can result in the trajectory of the aircraft becoming modified without the crew being notified.